158 THE MECHANISM OF LIFE 



(a thing that many people before must have observed, but did 

 not think about), and by-and-by Joule estimated how much 

 mechanical work was transformable into just how much heat, 

 and he measured this quantity of heat by noting the increased 

 temperature it could impart to a known mass of water. Joule's 

 work had enormous influence on physiology, for it became 



• possible to estimate the " calorific values " of known quantities 

 of various kinds of food substances. Now a certain mass of 

 combustible material thus became associated with a certain 

 quantity of mechanical work theoretically deducible from it, and 

 so it could be assumed that when food substances were eaten 

 and oxidised in the animal body, some of their heat became 

 transformed into mechanical work. Thus chemical energy could 

 pass into the form of heat, and heat could be transformed into 



} mechanical work. Apparently the animal body was a thermo- 

 dynamic machine. 



Let us note, very shortly, the other great ideas borrowed by 

 physiology from chemical and physical science. Graham, about 

 the middle of the nineteenth century, found that there were two 

 categories of chemical substances — the crystalloids (like common 

 salt) that could pass through the pores of an animal membrane 



' when they were dissolved in water, and the colloids (like gelatine 

 or albumen solutions) which could not pass through. Later on 

 de Vries showed how to measure osmosis — that is, the passage 

 of water through organic membranes. When a vegetable cell is 

 placed in pure water the latter passes through the wall, so that 

 the cell swells up, while, if it is placed in salt solution, which is 



■ more concentrated than the sap, it shrinks, because water now 

 passes out from the cell to dilute the stronger salt solution. 

 Graham's research on dialysis and de Vries' work on osmosis have 

 had a profound effect on physiological research — so much so that 



I it has been said that the chemistry of life is largely a matter of 

 the chemistry of colloids. Another most fruitful conception 

 was that of catalysis. It had long been known that fermenta- 



' tions in vegetable and animal substances were due to enzymes 

 (or ferments); that an exceedingly small quantity of the latter 

 substances could produce a chemical change which would not 

 otherwise take place; and that the ferment itself need not be 

 used up dxiring the reaction. But during the nineteenth century 

 it was discovered that a great number of purely mineral sub- 

 stances could act in precisely the same way with other mineral 



